Climatic interpretation of the Luochuan and Lingtai loess sections, China, based on changing iron oxide mineralogy and magnetic susceptibility

摘要

Hematite and goethite, estimated from diffuse reflectance spectra, and magnetic susceptibility (MS) have been determined from both the Luochuan and Lingtai loess sections of the Loess Plateau of central China. Changing concentrations of these iron oxides and MS were used to determine paleoclimatic change. Based on a model that relates pedogenic production of hematite and magnetite to precipitation, we propose two precipitation phases on the Loess Plateau during the last ～2.6 Ma. From the Wucheng-Red Clay boundary up to paleosol S5 (MIS 13), precipitation on the Loess Plateau was moderate, probably not frequently exceeding 450 mm/yr, and highly seasonal. From S5 to the present, precipitation during interglacial times has gradually increased to the present levels of 600-650 mm/yr for the localities examined. Throughout the Pleistocene, spanning both glacial and interglacial conditions, hematite to goethite ratios display a consistent decrease. This decrease suggests that precipitation on the Loess Plateau has been getting less seasonal and/or has been occurring during times when temperature was lower. In addition to these long-term changes, it also appears that iron oxide records from the Loess Plateau exhibit 400-500-ka cycles from the Wucheng-Red Clay boundary to S5. These 400-500-ka cycles may be analogous to similar length cycles recorded in δ~(13)C marine records in the South China Sea and may be linked through weathering and silica production as a result of the Asian monsoon.